The invention relates to a multipage teletext decoder for capturing and storing a plurality of teletext pages. The invention also relates to a television signal receiver provided with such a multipage teletext decoder. Such a receiver may be, for example, a television set or a video recorder.
A multipage teletext decoder of the type mentioned in the opening paragraph is described in the article "New Multipage Teletext Decoders" published in IEEE Transactions on Consumer Electronics, Vol. 37, no. 3, August 1991, pp. 441-448. As described in sections 6 and 7 of this article, the teletext decoder has a page memory for storing teletext pages, at least one acquisition circuit for capturing and storing a teletext page with a supplied request number, means for fixing a plurality of request numbers of pages to be captured, and a control circuit for applying a request number to the acquisition circuit.
The request numbers constitute the page numbers of the teletext pages which are to be captured and stored. The stored pages can subsequently be consulted by the user without any waiting time. The request numbers are programmed, for example, by the user and stored in a page number memory. They may also be determined by the transmitter. This transmitter transmits for each page the numbers of the pages most likely to be chosen as the next few pages. The way in which the request numbers are determined and fixed is irrelevant to the invention.
As described in the article quoted above, the control circuit applies a new request number to the acquisition circuit when a teletext page is received and stored. In this way all requested teletext pages are successively captured and stored. The entire storage procedure is repeated each time so as to achieve that stored pages are refreshed.
Teletext pages are transmitted in a repetitive cycle. Theoretically, all requested pages can be captured and stored within one such cycle. However, one of the problems in practice is that selecting the next request number and applying it to the acquisition circuit takes a relevant period of time. More particularly, the control circuit is coupled to the acquisition circuit by means of a relatively slow serial bus. Consequently, some time will elapse between the reception of a page and the actual start of the acquisition of a subsequent requested page. This "dead time" is so long that the next page to be captured may already have passed. It will then take a full cycle before the requested page is captured. In this case the storage of all requested pages may last many cycli. As a result, it takes a long rime before all the pages are available for immediate display. Moreover, the stored pages are refreshed less frequently, with the result that a page may contain out-of-date information upon display.
Generally, teletext pages are transmitted in a neat numerical order with ascending page numbers. It is not unusual that pages are clustered in groups and that "gaps" in the page numbering occur between the groups. For example, a group of home news pages comprises the numbers 103-107 and a subsequent group of foreign news pages comprises the numbers 112-118. The above-mentioned problem does not only occur if two teletext pages having consecutive page numbers must be stored (for example, 103 and 104), but also if two pages to be captured are located at either side of a gap ( for example, 107 and 112).
The sequence of the page number may also be interrupted regularly by "interrupted sequence" pages. These are pages whose page number does not fit in the numerical sequence. It is, for example, common practice to transmit frequently consulted pages several times per cycle. Such a page is then not only transmitted at its numerically logical location in the transmission cycle, but also elsewhere in the cycle. Thus, said problem may also occur if the numbers of two pages to be captured are far apart, but appear to succeed each other immediately in the transmission cycle.